1. Field of the Invention
The present invention relates to a method for manufacturing a MEMS sensor on a same single-crystal silicon wafer, and methods for fabricating a thin film of such MEMS sensor with improved etching processes.
2. Description of Related Art
Micro-Electro-Mechanical Systems (MEMS) is a high technology rapidly developed in recent years. MEMS components can be manufactured by advanced semiconductor manufacturing processes to realize mass production. Compared with the traditional electronic components, the MEMS components are more competitive in profile, power consumption, weight and price etc.
Usually, a MEMS condenser includes micro structures such as a thin film, a mass block and a cantilever beam etc. The traditional method for manufacturing such thin film uses surface sacrificial processes which include steps of: (i) fabricating a sacrificial layer by a Low Pressure Chemical Vapor Deposition (LPCVD) process or a Plasma Enhanced Chemical Vapor Deposition (PEVCD) process or a Physical Vapor Deposition (PVD) process; (ii) fabricating a thin film on the sacrificial layer by the fore deposition methods; and (iii) etching the sacrificial layer under the thin film to release the thin film to be movable micro structures. Such method can be used to fabricate a polysilicon thin film, a metal thin film or a medium thin film etc. However, such method is not suitable to fabricate a single-crystal silicon film.
Pressure sensors are those MEMS sensors earliest appear to be used. The pressure sensors are divided into a piezoresistive type, a capacitive type and a piezoelectric type etc. The piezoresistive pressure sensor has advantages of mass output signals, simple follow-up processing and easy for mass production. However, the piezoresistive pressure sensors are usually fabricated on the single-crystal pressure-sensitive silicon film. In mass production, it is a key guideline to keep the uniformity thickness of the pressure-sensitive silicon films of the piezoresistive pressure sensors. The current method for fabricating the pressure-sensitive silicon film is to anisotropically etch the single-crystal silicon wafer from its bottom side via a kind of alkaline liquor. As a result, a back chamber is formed at the bottom side of the single-crystal silicon wafer, and meanwhile, the pressure-sensitive silicon film is formed at the top side of the single-crystal silicon wafer. In order to control the thickness of the pressure-sensitive silicon film, a time controlling method is selected. However, such method can not uniform the thickness of the inside and outside pressure-sensitive silicon films. Another method is to use the highly doped silicon film to control the thickness of the pressure-sensitive silicon film. However, since the piezoresistances cannot be fabricated on the highly doped silicon film, such method is not suitable to manufacture the pressure-sensitive silicon films of the piezoresistive pressure sensors. Another method is to use electrochemical etching to achieve the lowly doped silicon film which can be used to fabricate piezoresistances. However, such method needs additional apparatus, such as expensive potentiostats and clip tools for protecting the silicon wafer. The cost is accordingly enhanced and the manufacture efficiency is decreased due to additional processes.
Acceleration sensors are other kinds of MEMS sensors and are divided into a piezoresistive type, a capacitive type and a piezoelectric type etc. The piezoresistive type acceleration sensor needs to fabricate piezoresistances on its cantilever beam in order to detect the acceleration. Usually, the cantilever beam is fabricated from a single-crystal silicon film, which still meets the problems described in the above piezoresistive pressure sensors.
Hence, it is desired to have improved methods for manufacturing a MEMS sensor and its thin film.